Galactic Origin Models A Case For Galactic R. Subrahmanyan & R. - PowerPoint PPT Presentation

Jack Singal RSB Workshop University of Richmond July 2017 Tests of Simple Galactic Origin Models

A Case For Galactic R. Subrahmanyan & R. Cowsik, 2013, ApJ , 776, 42 " Is there an unaccounted excess Extragalactic Cosmic Radio Background? " They use a 2-component 5-parameter vs frequency Galactic spatial model for radio emission to get a high halo component which could give the RSB surface brightness.

Some Cases Against Galactic (to be addressed in turn) • Inverse-Compton • Observations of other galaxies like ours • Large scale Galactic radio emission is well fit by a csc(b) spatial model, and a correlation of radio with C+ emission at 158 µm agrees

Inverse-Compton • J. Singal, L. Stawarz, V. Petrosian, & A. Lawrence (2010, MNRAS , 409, 1172) - Synchrotron emissivity is a combination of electron energy density (U e- ) and magnetic field energy density (U B ) - For a given observed synchrotron level 𝜉 𝑡𝑧𝑜 𝑉 𝜉 𝑡𝑧𝑜 if U B ↓ then U e- ↑ - If U e- ↑ then inverse -Compton upscattering by these electrons increases

Inverse-Compton • Singal et al. (2010, MNRAS , 409, 1172) The electrons that produce the radio emission through synchrotron cannot overproduce another background via inverse-Compton – places a lower limit on the magnetic field in the emitting regions 1 nG Observed EBL 1 μ G

Inverse-Compton • The same X-ray inverse-Compton argument applies to our Galactic halo, where FR measures show the mag field is ~1 μ G (Taylor et al, 2009, ApJ , 702, 1230) but optical/UV flux is higher than in intergalactic space 1 nG Observed EBL 1 μ G

Other Galaxies We can see whether galaxies similar to ours have a large bright radio halo - J. Singal, A. Kogut, E. Jones, & H. Dunlap, 2015, ApJL , “Axial Ratio of Edge-On Spiral Galaxies as a Test For Bright Radio Halos .” 799 , L10 • There are radio emission contour maps of galaxies nearby enough to be resolved and which are seen edge-on • Do they have a shape structure (round-ish halos bulging above and below the plane of the disk) that would be predicted by the Subrahmanyan & Cowsik model?

Other Galaxies We can see whether galaxies similar to ours have a large bright radio halo - Singal et al., 2015, ApJL , “Axial Ratio of Edge -On Spiral Galaxies as a Test For Bright Radio Halos.” 799, L10 • Do they have a shape structure (round-ish halos) that would be predicted by the Subrahmanyan & Cowsik model? 0.1 K Predicted contours at 0.03, 0.1, 0.3, 1, and 3 K for the SC model at 1.5 GHz. 1 K Contours near 0.1 K or below can distinguish the presence of a halo if the beam smearing is small enough. (It needs to be round-ish for there to be an SC halo).

Other Galaxies We can see whether galaxies similar to ours have a large bright radio halo All edge-on (>75º inclination) normal spirals mapped between 150 and 1500 MHz that have a contour that can distinguish the presence or absence of a spherical halo Absolute level comparison: Compares contours in observed maps to same absolute level contour in predicted SC model map Fractional level comparison: Compares contours in observed maps to contour at same fraction of peak brightness in predicted SC model map

Other Galaxies We can see whether galaxies similar to ours have a large bright radio halo - Singal et al., 2015, ApJL , “Axial Ratio of Edge -On Spiral Galaxies as a Test For Bright Radio Halos.” 799, L10 We use all radio contour maps of nearby edge-on SC model prediction spiral galaxies with a contour capable of distinguishing the presence of a bright spherical halo at the level of the SC model Absolute comparisons compare absolute levels galaxy map data in the model with those observed – insensitive to structure in the plane but potentially sensitive to normalization of galaxy brightness. SC model prediction Fractional comparisons are insensitive to normalization of brightness but potentially sensitive to structure in the plane. galaxy map data Galaxies similar to the Milky Way are more elliptical than the SC model, disfavoring this type of large radio halo

Other Galaxies We can see whether galaxies similar to ours have a large bright radio halo - Singal et al., 2015, ApJL , “Axial Ratio of Edge -On Spiral Galaxies as a Test For Bright Radio Halos.” 799, L10 Galaxies similar to the Milky Way are more SC model prediction elliptical than the SC model, disfavoring this type of large radio halo galaxy map data To modify the SC halo to fit the observed data would require dimming it by a factor of four. This would place most of the isotropic signal into a non-halo (presumably extragalactic) SC model prediction component. galaxy map data

Two Galactic Emission Models How to distinguish (from maps alone) between Galactic monopole and extragalactic component? 1) Model Galactic radio emission with some function of latitude 2) Correlate Galactic radio emission with some other emission of known spatial structure See Kogut et al. (2011, ApJ , 734, 4)

csc(|b|) Average sky temp in pixels at latitude (b) vs. csc(|b|) for b >10º Haslam Error bars x 20 Scatter about lines comes from higher order spatial structure ARCADE 2 To get Galactic polar cap emission temperature, extrapolate line to 90º Error bars x 5 latitude

Radio C+ correlation Idea: find a Galactic emission map with a well defined high latitude temperature to correlate with our spatial maps - Needs to be measured over full sky - Well defined absolute level along every line of sight (WMAP is out) - Doesn’t suffer from extinction (Thermal dust and Hα are out) - Needs to be present in highly ionized regions (21 cm map is out) If only such a thing existed… It does! The FIRAS C+ emission map (158 μ m)

Radio C+ correlation (2) Haslam 408 MHz vs. C+ ARCADE 2 3.15 GHz vs. C+ Breaks into two distinct regions. One near Galactic center (upper) and one near Cygnus arm (lower). To get polar cap temp at a frequency: Multiply the C+ polar cap intensity by the slope of the lower line

They both agree So with a Galactic polar cap temperature one can calculate a Galactic monopole term

More Local? What about the Local Bubble? (A region that the sun and nearby stars are passing through that has hot x-ray emitting gas and a low density compared to the average ISM) • It would be highly polarized since the Galactic magnetic field is locally pretty constant. In that case, we would expect to see some quadrupole polarization structure in WMAP 22 GHz aligned with the local Galactic magnetic field, but we don’t. • We don’t see similar localized radio bubbles in other places

Conclusions • A very radio bright Galactic Halo would lead to a number of issues: • The x-ray background would be overproduced via inverse- Compton unless strangely high magnetic fields were present in the halo • Our Galaxy would be quite anomalous compared to similar ones • There would have to be anomalous radio emission that does not correlated with C+ emission